1.2 Outer Respiration (basic processes of lung respiration and lung volumes)

The concept of this topic is just how we take air into our lungs, how much we can take in and what happens when it is inside our lungs. Simple.

So, lets start by pointing out the difference between outer respiration and inner respiration. The outer respiration is easy to understand as it is taking in air from the outside (this is also known as breating). Inner respiration is the use of oxygen in the mitochondria to produce ATP (it is being carried out inside the cells).

Now, the most important organ in this story of outer respiration is, of course, the lungs.

The question is how do they work? Its kind of simple actually. The chest expands making a vacuum. Air flows in; simple. For expiration; the muscles simply relax and the elasticity of the lungs just pushes the air out. During heavy breathing, however, muscles can help this process as well.

One point to note here is that during inspiration, external intercoastals do the work and during expiration, internal intercoastals do the work; so they are opposites.

The intrapleural pressure is always negative -4 to -9 mBar. So, if we want to define transpulmonary pressure (the pressure across the chest cavity) we can use the alveolar pressure and intrapleural pressure.

Now study of anesthesiology is complete without clinical parameters. So lets now define some of them.

1. The first one on the list is the tidal volume (TV) which is during normal breathing. Think of this like smooth calm small tides on a beach. The tidal volume is the volume of air breathed in and out during normal resting breathing.
2. Now, in the next phase like during exercise we have two options: we can take a deep breath in or force all the air out of our lungs. Okay, so if we take a deep breath in, we can take in almost 3 liters of air. I memorized this by thinking that our chest expands 3x during this process. This is known as the Inspiratory Reserve Volume (IRV).
3. If we force all of the air out of the lungs, to the best of our ability, we can expel around 1 liter of air. This is the Expiratory Reserve Volume (ERV).
4. But, even after we try to force all the air out of our lungs, there still isn't a vacuum in our lungs right? That's the Functional Reserve (FR). This is important because if all the air in the lungs is expelled, the alveoli would collapse and it is very hard to open them back up. This is around 1.2 liters. So a little bit more than ERV.

Lastly, the most important aspect of breathing (outer respiration) is diffusion. It can be defined by Fick's Law of diffusion which states that diffusion is directly proportional to the surface area across which it is occuring and the diffusion gradient and inversely proportional to the distance the gases have to pass through. Simple and it makes sense. If there is more surface area, then more diffusion thats why the alveoli in our lungs make up a huge surface area. If the diffusion gradient is big, easier diffusion. But! If there is a large distance the gases have to travel to diffuse, it will not be effective.

And that's it!